In the 2006 film Children of Men, the world is devastated by plagues. The UK has become a prison state, regularly executing immigrants. The future of humanity is at stake. No child has been born for 18 years.

A far-flung fiction? Not so, says new, extensive, and eye-opening research.

Experts found that the world population is likely to peak at around 9.7 billion people in 2064, before falling to 8.8 billion in 2100.

"These forecasts suggest good news for the environment, with less stress on food production systems and lower carbon emissions," said the University of Washington's Christopher Murray, who led the study.

Previous models had predicted that the world's population would continue to increase throughout the 21st Century. But, as societies modernise and women are empowered to live their own lives, fertility rates drop sharply.

Maintaining population levels requires 2.1 children per household. However, "as women become more educated and have access to reproductive health services, they choose to have less than 1.5 children on average," said Murray.

This means that 183 out of 195 countries studied will be shrinking by the end of the century. Without major shifts in behaviour or mass immigration, major economies like China, Japan, Spain, and Poland could all see their populations halve.

Sub-Saharan Africa will buck the trend and benefit from great economic opportunity, with Nigeria set to become the world's second-most populous nation (after India), with its population rocketing to 800 million. This will cause "radical shifts in geopolitical power", says Lancet editor Richard Horton, who published the study.

For most of the world, there will be a huge swing towards older populations. Ageing societies, while typically more peaceful, are not thought to be particularly desirable. Throughout history, the most prosperous and powerful nations have been those rich in young people.

When population pyramidsA population pyramid, also called an "age-gender-pyramid", is a graphical illustration that shows the distribution of various age groups in a population (with the youngest at the bottom). This forms the shape of a pyramid when the population is growing. are flipped onto their heads - with the old outnumbering the young - the most important question becomes: who will make the money? One answer is immigrants. As UCL's Professor Ibrahim Abubaka told the BBC: "The distribution of working-age populations will be crucial to whether humanity prospers or withers."

For other thinkers, a shrinking population can only be a good thing. In 1798, Thomas MalthusA clergyman who was interested in maths and political thought and became famous for his extremely influential work An Essay on the Principle of Population. Predictions about population growth leading to food shortages are known as "Malthusian" after him. described how humanity was condemned to outgrowing Earth's resources and living in eternal misery. Martin Luther King referred to "the modern plague of overpopulation", while Mikhail Gorbachev suggested that "the ecological crisis, in short, is the population crisis".

Yes. It is good news for the environment. Once this happens, the whole world will be in a position to control its own destiny. As the late Hans RoslingSwedish physician, academic, and public speaker (1948-2017). put it, any further growth in population becomes a question of "love" not necessity.

No. For thousands of years, the young have outnumbered the old. Reversing this trend could have terrible consequences. Economies could collapse without enough workers. Societies run by old people could become highly risk-averse and backward looking. Cultural life and entertainment could
stagnateStop developing; become inactive or dull. Usually to describe water or air, when it ceases to flow or move.. It would be awful!